Drought-weary California, heading into a long, hot summer of water shortages and extreme fire risk, received some potentially good news Thursday: Federal scientists announced there is now a 4-in-5 chance of El Niño conditions developing by the end of the year.

El Niño events -- when warmer-than-normal sea surface temperatures in the Pacific Ocean at the equator affect the jet stream -- can lead to wetter winters in California.

Citing a huge mass of warm water that continues to move east toward South America, the National Oceanic and Atmospheric Administration increased its probability for El Niño developing next winter to 78 percent, up from 66 percent last month, and 36 percent in November.

"We are now even more bullish that an El Niño is impending," said Michelle L'Heureux, a meteorologist with the NOAA's Climate Prediction Center in College Park, Md.

But there's no guarantee California's persistent drought will be over in six months. Generally speaking, the warmer the ocean water, the increased likelihood of heavy rainfall during El Niño years. During mild El Niño years, when the ocean water is only slightly warmer than historic averages, there are just as many drier-than-average years as soaking ones.

"There are all kinds of El Niños: small, medium, large and Godzilla," said Bill Patzert, a research scientist and oceanographer at NASA's Jet Propulsion Laboratory in Pasadena.

"I don't see the Godzilla,'' he said. "But I'll give it another couple of months. This still could be El Fizzle. I don't want to recommend that you invest any of your retirement in the umbrella market yet."

Despite some healthy downpours that California received in March, on Thursday the Sierra Nevada snowpack, a key source of water for farms and cities, was 13 percent of the historic average for this date. Most major cities received half or less of their average rainfall this year.

San Jose, with 6.32 inches since last July 1, was at 43 percent of average rainfall. Oakland, 50 percent. And San Francisco, 54 percent.

Major reservoirs, hamstrung by 2013 being the driest year in recorded California history, are mostly at about half of their historic average for the beginning of summer.

Leaders at California water districts are privately hoping that the El Niño shaping up in the Pacific Ocean now will save them from what will be dire circumstances with widespread water rationing next year if the upcoming winter is unusually dry again, creating a fourth dry year in a row.

But they continue to say they aren't counting on El Niño to save the state.

"It would be a mistake to think that an El Niño preliminarily forecasted for the upcoming winter season will alleviate California's multi-year drought," said Joan Maher, deputy operating officer for the Santa Clara Valley Water District.

The district, which provides water and flood protection to 1.8 million people from Palo Alto to Gilroy, last month doubled the rebate amounts that it offers to people who take water saving steps between now and September. For example, the agency will now pay $2 per square foot for homeowners who replace lawns with drought-tolerant plants, and $200 for homes that install a graywater system to use laundry water for irrigating landscaping.

"We must take a cautious view and reduce water use to get through this year and prepare for next year if the drought continues," Maher said.

NOAA scientists, who glean information from buoys and satellites, say that with each passing month, they will know more about how strong the emerging El Niño will be. Currently, computer models run by many of the world's top weather agencies estimate that sea surface temperatures will range from about 1 to 3 degrees Fahrenheit warmer than normal by next fall.

The last major, soaking El Niño event, in 1997, saw surface temperatures 5 degrees warmer than normal at some times. That led to major flooding and landslides, and 35 counties being declared disaster areas.

California water managers say the best case would be a fairly strong El Niño event that brings rainfall next winter to about 50 percent more than average.

"We want to see sustained, above-normal precipitation and above-normal snowpack, but not so over-the-top that you end up with major flooding," said Michael Anderson, the state climatologist with the California Department of Water Resources.